1. Field of the Invention
This invention relates to power transmission belts and, more particularly, to a belt in which fibers are embedded in the belt body and exposed at the belt flanks to minimize noise generation as the belt flanks engage and separate from a cooperating pulley. The invention is also directed to a method of forming the power transmission belt to increase the exposed surface area of the fibers at the belt flanks.
2. Background Art
V-ribbed belts are used in a number of diverse environments. Exemplary applications for V-ribbed belts are in automobiles and other types of vehicles, agricultural equipment, domestic electrical equipment, etc.
The typical V-ribbed belt has a plurality of longitudinally extending, parallel ribs which engage within correspondingly configured pulley grooves. The V-ribbed belt has a number of advantages over conventional V-belts. Most of the advantages of the V-ribbed belt stem from its thin construction. This not only gives the belt a compact configuration, but makes it highly flexible which allows it to be used in conjunction with small diameter pulleys. Because of the inherent flexibility of the V-ribbed belt, the bending stresses in the belt in use are minimized, which accounts for a long belt life. At the same time, the flexibility of the V-ribbed belt accounts for the system driving the belt consuming significantly less energy. A further advantage of the V-ribbed construction is that normally the ribs do not move radially into the cooperating pulley grooves as far as a conventional V-belt would. This reduces the frictional forces on the belt during operation and results in a prolonging of the belt life as well as additional energy savings.
The V-ribbed belt construction does, however, have some drawbacks. The V-ribbed belt is prone to slipping relative to cooperating pulleys if the tension on the belt becomes excessive and/or if there is an abrupt variation in load on the associated system. The result of this may be a shortened belt life as a result of the belt rubber being abraded or cracking or the belt otherwise wearing out.
Another problem with V-ribbed belt systems results from the shallower grooves required in the pulleys around which the belt is trained. Since the ribs do not have to extend radially inwardly into the grooves as far as with V-belts, the pulley grooves are commonly made of a relatively shallow depth. A result of this is that dust, sand, mud and other foreign matter encountered in the operating environment, tend to accumulate in the pulley grooves. This foreign matter may not only prevent full seating of the ribs in the pulley grooves, which could result in belt slippage, but also abrades the belt rubber in use which may shorten the belt life.
A still further problem with V-ribbed belts is that of noise generation as the belt ribs move into and out of engagement with the cooperating pulleys. Noise is also generated as the belt slips in a circumferential direction during system operation.
One proposed solution to the problem of noise generation has been the provision of a cloth layer on the belt ribs. The cloth layer does reduce noise generation and wear on the belt rubber, but introduces new problems. First, the application of the cloth layer involves an additional manufacturing step. This complicates manufacturing and increases the attendant costs. Further, the cloth itself is prone to failure and, once this occurs, continued use of the belt may become impractical.
An alternative to the provision of a cloth layer is the use of short fibers which are commonly sprayed onto the belt flank surfaces. While the use of sprayed on fibers does in fact reduce noise levels and abrasion on the belt body, the application of the fibers, as with the aforementioned cloth, complicates belt manufacture. Further, it is difficult to permanently adhere the short fibers to the belt flanks and, consequently, the fibers tend to fall off during use. The beneficial effect of the fibers may then not be consistently realized during the life of the belt. Further, the flexing characteristics of the belt are altered by the provision of each of the cloth and fibers.
A further proposed solution to the above problems has been the embedding of a fabric layer in the ribs during manufacture. This has proven impractical in V-belts since the ribs, in most applications, do not have sufficient depth to accommodate the fabric layer.
The V-ribbed belts of the prior art have been constructed in various different manners. One such method is disclosed in Japanese Patent Publication No. 52-15310. This publication discloses a matrix manufacturing method employing a cylindrical mandrel which is surrounded by a tubular, vulcanized, rubber forming sleeve. The sleeve has axially spaced, circumferential, V-shaped grooves integrally formed therein and corresponding to the desired end shape of the ribs of the V-ribbed belt. The belt components are sequentially built up onto the forming sleeve. In one exemplary belt construction, a flat rubber layer is initially wound onto the sleeve or, alternatively, that layer is molded in place. Tensile cords are then spirally wound around the rubber layer to press the rubber layer into partial conformance with the grooves. An additional rubber layer followed by a fabric layer are then laminated in place after which the belt components are vulcanized. The vulcanized belt sleeve is then cut to produce individual belts.
The matrix method of belt formation has inherent drawbacks. One of the biggest problems is encountered when the first rubber layer against the forming sleeve is not a molded layer, as for example when it is applied as a sheet layer. As the belt components are vulcanized, the rubber layer conforms fully to the ribs. As this occurs, the tensile cords tend to conform to the contour of the forming sleeve; that is, the tensile cords tend to move radially into the grooves and bend around the walls between the adjacent grooves. The tensile cord orientation thus deviates from the desired straight longitudinal direction. The irregular tensile cord pattern commonly results in an effectively bent/slackened tensile cord which straightens under load to allow undesirable elongation of the belt.
While molding of the first placed belt rubber layer against the forming sleeve eliminates the above problem, it also complicates manufacture. The number of belt forming steps is increased, which adds undesirably to the cost of the belt.
A further problem common to both matrix formation methods described above is that it is difficult to remove the vulcanized belt sleeve wrapped around the forming sleeve. Further, the life of the forming sleeve is inherently short due to the fact that it is repetitively subjected to severe conditions such as heating at high temperatures, high pressure compression, cooling, etc.
Another belt formation method is disclosed in Japanese Patent Publication No. 52-17552. In this publication, the ribs are formed by grinding. The belt components are sequentially built up onto a mandrel in an inside out arrangement. In one exemplary belt construction, the following layers are built up in sequence: fabric; a rubber layer; tensile cords; and an outer rubber layer which defines the innermost layer of the completed belt. Once all belt components are in place, the belt sleeve is vulcanized. A grinding wheel, having a plurality of circumferential V-shaped grooves, is urged against the outermost rubber layer to define the belt ribs. To accomplish this, both the belt sleeve and the grinding stone/grinding wheel are rotated about substantially parallel axes as they are moved, one against the other, at right angles to their rotational axes.
It is known to define V-ribbed belts with short, laterally extending fibers in the compression section. When the ribs are defined by conventional grinding techniques, such as that described above, the fibers are cleanly severed in the same plane as the belt flanks. There is a tendency of the grinding wheel to melt the rubber and in so doing cover the lateral-most surface of the severed fibers with rubber during this process. Consequently, little or no part of each fiber ends up being directly exposed at the belt flanks. The result is that the belt flank is effectively solid rubber which has a high coefficient of friction compared to the fibers and which binds as the ribs encounter the pulley surfaces and move away therefrom during operation. The result is undesirable noise generation, which was previously discussed.
While increasing the quantity of laterally extending fibers in the compression section overcomes the above problem, results in more exposed fibers at the belt flanks, and achieves the desired result of decreasing the frictional coefficient, the quantity of fibers necessary to accomplish this may compromise the integrity of the belt compression section. Further, as the number of fibers increases, it becomes more difficult to uniformly distribute the fibers. The flexibility and life of the belt may also be compromised. Increasing the number of fibers can create points of weakness in the rubber in which the fibers are embedded.
Another known method of dealing with the above noted problems is disclosed in Japanese Patent Publication No. 58-34697. In this publication, a belt sleeve is disclosed with an inner, expansible canvas layer, which is formed together with the ribs during manufacture.
The difficulty with the above method is that the canvas employed is expandable readily only in a single direction. If the canvas is arranged to be expandable in the longitudinal direction of the belt, it is difficult to conform the canvas transversely thereto around the individual ribs. If the canvas is aligned to be expandable laterally of the belt, while this facilitates formation of the canvas around the ribs, flexibility of the belt is deteriorated. The result is excessive heat generation and possible cracking as the belt is flexed during operation.